Split creation, configuration and use of FileLister objects

The constructor now has zero arguments. showDirectories and showFiles
are now set using setter methods.

FileLister::browse is now the sole way to start a scan for files and
directories, replacing the initial path in the constructor and the
setPath method. It allows merging results from multiple directories
as before.

This is all to make explicit how many times the costly task of browsing
a directory is actually carried out.
Pre-process the filter string in FileLister::setFilter

Instead of splitting the filter for every file in FileLister::browse,
the filter is split immediately in FileLister::setFilter, improving
performance.

getFilter is removed because it was unused. This also removes the need
to update the return type to 'const std::vector&' in the method and
rewrite its callers.
Optimise duplicate checking and sorting in FileLister::browse

Using a std::set makes both duplicate checking and sorting run in
O(n log n).